Ultrasonic welding is an industrial process involving high frequency ultrasonic acoustic vibrations that are locally applied to workpieces being held together under pressure to create a solid-state weld. This process has applications in the electrical/electronic, automotive, aerospace, appliance, and medical industries and is commonly used for plastics and especially for joining dissimilar materials. Ultrasonic welding of thermoplastics results in local melting of the plastic due to absorption of vibration energy. The vibrations are introduced across the joint to be welded. In metals, ultrasonic welding occurs due to high-pressure dispersion of surface oxides and local motion of the materials. Vibrations are introduced along the joint being welded.
U.S. Pat. No. 8,950,458, incorporated herein by reference, provides an example of an ultrasonic welding system. Ultrasonic welding systems typically include a press to apply pressure to the two parts to be assembled under pressure, a nest or anvil where the parts are placed for allowing high frequency vibrations to be directed to the interfaces of the parts, an ultrasonic stack that includes a converter for converting the electrical signal into a mechanical vibration, an optional booster for modifying the amplitude of the vibration, a sonotrode or “horn” for applying the mechanical vibration to the parts to be welded, an electronic ultrasonic generator or power supply delivering a high power AC signal, and a controller for controlling the movement of the press and the delivery of the ultrasonic energy.
The power supply provides high-frequency electrical power to the piezoelectric-based transducer, creating a high-frequency mechanical vibration at the end of the transducer. This vibration is transmitted to the sonotrode (e.g., via the booster) which transmits the vibrations to workpieces. The workpieces, usually two thin sheets of metal in a simple lap joint, are firmly clamped between the sonotrode and a rigid anvil by a static force. The top workpiece is gripped against the moving sonotrode by a knurled pattern on the sonotrode surface. Likewise, the bottom workpiece is gripped against the anvil by a knurled pattern on the anvil. The ultrasonic vibrations of the sonotrode, which are parallel to the workpiece surfaces, create the relative friction-like motion between the interface of the workpieces, causing the deformation, shearing, and flattening of surface asperities.
Welding system components, commonly referred to as the transmission line or “stack”, are typically housed in an enclosure case that grips the welding assembly at critical locations (most commonly the anti-node) so as to not dampen the ultrasonic vibrations, and to provide a means of applying a force to and moving the assembly to bring the sonotrode into contact with the workpieces and apply the static force.
Ultrasonic welding systems typically include only one transducer or converter situated on one side of the sonotrode. In order to boost power, attempts have been made to use multiple converters. However, using more than one converter can lead to difficulties keeping the vibrations synchronized and in phase with one another.
U.S. Pat. No. 7,984,839 to Büttiker discloses an ultrasonic device with longitudinal and torsional sonotrodes and two converters operated by a generator. US 2015/0210003 to Short et al. discloses a transverse sonotrode for ultrasonic welding which, in some cases, may include a second transducer. However, neither Büttiker nor Short is concerned with ensuring that the converters are in phase.
U.S. Publication No. 2016/0067912 to Schneider discloses an ultrasonic welding apparatus with a plurality of sonotrodes and a plurality of converters. In order to avoid an undesirable interaction occurring between the two sonotrodes, the converter excites the first sonotrode at a frequency fs1 the second sonotrode at a frequency fs2, fs2 being greater than fs1. Yet, Schneider does not disclose ensuring that multiple converters are in phase with one another.
It is therefore desired to provide an ultrasonic welding system with high power delivered in an effective way such that power is delivered in phase.